A typical wireless network includes a number of base stations each radiating to provide coverage in which to serve wireless communication devices (WCDs) such as cell phones, tablet computers, tracking devices, embedded wireless modules, and other wirelessly equipped devices. In turn, each base station may be coupled with network infrastructure that provides connectivity with one or more transport networks, such as the public switched telephone network (PSTN) and/or the Internet for instance. With this arrangement, a WCD within coverage of the network may engage in air interface communication with a base station and may thereby communicate via the base station with various remote network entities or with other WCDs served by the base station.
Depending on the specific underlying technologies and architecture of a given wireless communication network, base stations may take different forms. In a code division multiple access (CDMA) system configured to operate according IS-2000 and IS-856 standards, for example, a base station may include a base transceiver system (BTS) under the control of a base station controller (BSC). In a universal mobile telecommunications system (UMTS) configured to operate according to ITU IMT-2000 standards, the base station is usually referred to as a NodeB, and is usually under the control of a radio network controller (RNC). In a UMTS network configured to operate to Long Term Evolution (LTE) standards, evolved NodeBs (eNodeBs) may communicate directly with one another, while under functional coordination of a mobility management entity (MME). Other base station architectures and operational configurations are possible as well.
Further, a wireless network may operate in accordance with a particular air interface protocol (i.e., radio access technology), with communications from the base stations to WCDs defining a downlink or forward link and communications from the WCDs to the base stations defining an uplink or reverse link. Examples of existing air interface protocols include, without limitation, wireless wide area network (WWAN) protocols such as Orthogonal Frequency Division Multiple Access (OFDMA (e.g., Long Term Evolution (LTE) and Wireless Interoperability for Microwave Access (WiMAX)), Code Division Multiple Access (CDMA) (e.g., 1×RTT and 1×EV-DO), and Global System for Mobile Communications (GSM), and wireless local area network (WLAN) protocols such as IEEE 802.11 (WIFI), BLUETOOTH, and others. Each protocol may define its own procedures for registration of WCDs, initiation of communications, handover or handoff between coverage areas, and other functions related to air interface communication.
In practice, a base station, such as an eNodeB, may be configured to provide service to WCDs on multiple carrier frequencies or “carriers.” Each carrier could be a time division duplex (TDD) carrier that defines a single frequency channel multiplexed over time between downlink and uplink use, or a frequency division duplex (FDD) carrier that defines two separate frequency channels, one for downlink communication and one for uplink communication. Each frequency channel of a carrier may then occupy a particular frequency bandwidth (e.g., 1.4 MHz, 3 MHz, 5 MHz, 10 MHz, or 20 MHz) defining a range of frequency at a particular position (e.g., defined by a center frequency) in a radio frequency band (e.g., in the 800 MHz band, the 1.9 GHz band, or the 2.5 GHz band).
Each carrier may also define various logical channels to facilitate communication between the base station and one or more served WCDs. For instance, on the downlink, a carrier may define a reference channel on which the base station broadcasts a reference signal useable by WCDs to detect and evaluate coverage, various other downlink control channels to carry control signaling (such as resource-scheduling directives) to WCDs, and one or more shared or traffic channels for carrying bearer data (e.g., user or application level data) to WCDs. And on the uplink, a carrier may define one or more uplink control channels to carry control signaling (such as resource scheduling requests, channel state reports, and the like) from WCDs, and one or more shared or traffic channels for carrying bearer data from WCDs. In practice, the shared or traffic channels may define particular physical resources for carrying data between the base station and WCDs.
When a WCD enters into coverage of a base station, the WCD may attach, register, or otherwise associate with the base station, and the base station may then serve the WCD on one or more carriers. The base station may then be referred to as the WCD's “serving” base station. Herein, the term “serving” will, in general, be used to describe a particular base station as such only when it is not otherwise apparent from context. In practice, the process of serving the WCD may involve the serving base station allocating use of particular air interface resources, such as traffic channels or portions thereof, to carry data communications to and from the WCD, and managing transmission on those resources, such as controlling what modulation scheme is used for the transmissions.
For instance, when the serving base station has data to transmit to the WCD, the serving base station may select certain downlink resources to carry the data and may determine a modulation scheme for transmission on those resources, and the base station may then (i) transmit to the WCD a scheduling directive instructing the WCD to receive the data on the scheduled resources using the determined modulation scheme, and (ii) transmit the data on the indicated downlink resources using the determined modulation scheme. Likewise, when the serving base station receives from the WCD a request for the WCD to transmit data to the base station, the base station may select certain uplink resources to carry the data and may determine a modulation scheme for transmission on those resources, and the base station may then (i) transmit to the WCD a scheduling directive instructing the WCD to transmit the data on the scheduled resources using the determined modulation scheme and (ii) receive the transmission from the WCD accordingly.
A WCD may also move between neighboring coverage areas of base stations. More specifically, as a WCD moves between wireless coverage areas of a wireless communication system, or when network conditions change or for other reasons, the WCD may “handover” (or “hand off”) from operating in one coverage area (e.g., a serving coverage area) to operating in another coverage area. In a usual case, this handover process is triggered by the WCD monitoring the signal strength of various nearby available coverage areas, and the serving base station (or some other controlling network entity) determining when one or more threshold criteria are met. For instance, a WCD may continuously monitor signal strength from various available coverage areas and notify its serving base station when a given coverage area has a signal strength that is sufficiently higher than that of the serving base station. The serving base station (or some other controlling network entity) may then direct the WCD to handover to the base station of the given coverage area. By convention, a WCD is said to handover from a “source” base station (or source coverage area) to a “target” base station (or target coverage area). At the time that a handover is triggered, the source base station is the WCD's serving base station.